1. Field of the Invention
This invention relates generally to robots, and more particularly to a robot with a multi-jointed arm.
2. Description of the Prior Art
Higher speed and greater precision in the motion of robots, such as multi-jointed robots utilized in assembly lines, are important, since they result in shorter "tact-time" and higher productivity. The term "tact-time" refers to the time during which a robot performs its assigned operation. Conventional speed reducers, such as harmonic drive gears, often are poor in rigidity, exhibit back-lash, and have narrow design limits with respect to input revolution rate and speed reduction ratio. Therefore, direct driving of arms using low-speed, high-torque motors without speed reducers has been developed. However, if a motor is arranged far from the pivotal center on an arm, an excess moment of inertia is added to the arm due to the mass of the motor. Therefore, it is preferable that the motor be arranged at the pivotal center of the arm.
Japanese Patent Disclosure (Kokai) 60-76992 discloses a directly driven multi-jointed robot. In the disclosure, a first arm pivots in response to a first motor. A second arm, which is attached to a shaft passing through an aperture in the first arm, is driven by a second motor with a double parallel link mechanism. The second motor is set at the pivotal center of the first arm. A single crank shaft, which is directly driven by the second motor, drives the double parallel link mechanism. However, since the crank shaft is driven at one side of the second motor, the crank shaft is highly stressed. Thus, the shaft must have a high strength and rigidity.
Furthermore, since the second motor is supported on the first arm, the mass and the inertia of the second motor are added to the first arm, and the reaction torque of the second motor is added to the first motor load. Therefore, the first motor requires large torque and large power.